HYBRID NUTRITIONAL REQUIREMENTS 117 



of partial deficiencies could be cited. Their effect is to decrease the rate of 

 growth but not to inhibit it entirely. 



As a result of investigations which have extended over the past decade or 

 two, we know of many examples in which poor growth or failure to grow in a 

 specific environment is due to the inability of the organism to synthesize 

 adequate quantities of one or more essential metabolites. The metabolic 

 machinery lacks a part, or some part works slowly, with the result that the 

 organism does not make sufficient quantities of one or more growth essen- 

 tials, and unless supplied with the missing materials from without, grows 

 slowly, or not at all. 



Not all instances of failure to grow or of poor growth in a given environ- 

 ment are explainable on the basis of deficiencies of essential metabolites. In 

 some instances growth may be limited by autogenic growth inhibitors. 



AUTOGENIC INHIBITORS 



Zalokar (1948), Emerson (1947, 1948), and others have described a mutant 

 strain of Neurospora which grows poorly at high temperatures. Growth oc- 

 curs if sulfonamide is added to the medium. One might conclude that 

 sulfonamide acts for this organism as an essential metabolite. It appears, 

 however, that this mutant produces growth inhibitors which are antagonized 

 in some way by the sulfonamide. This seems to be an example of poor growth 

 caused by the accumulation of autogenic growth inhibitors, and not because 

 of the lack of an essential metabolite. 



Information on the role of autogenic inhibitors in limiting growth is less 

 specific and more difficult to obtain than evidence for the limitation of growth 

 due to a deficiency of an essential metabolite. How commonly do internally 

 produced inhibitors reduce growth? What is the nature of these substances? 



From the investigation of antibiotic substances we know that many organ- 

 isms form metabolic products, highly inhibitory for organisms other than 

 themselves. Do they also produce substances which limit their own growth? 

 The role of autogenic inhibitors in limiting growth deserves much more 

 attention than it has received. 



It is well known that minute amounts of specific chemical compounds 

 materially modify the amount and nature of growth in plants. Zimmerman 

 and Hitchcock (1949) treated Kalanchoe plants with small amounts of the 

 ortho, para, and meta forms of chlorophenoxyacetic acid. The para form 

 caused the apical meristem to develop into a spathe-like organ which could 

 be cut off and rooted. It had little resemblance to Kalanchoe. The ortho and 

 meta forms of this compound did not have this effect. This modification was 

 not a mutation. The effect wore off as the chemical in the plant disappeared, 

 and the Kalanchoe eventually returned to its normal growth pattern. If the 

 change had been permanent, we would have been inclined to call it a muta- 

 tion and look for a genie explanation ; i.e., look for a gene which controlled the 



